2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 284-7
Presentation Time: 9:40 AM


NG, G.-H. Crystal, Department of Earth Sciences, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455-0231, BEKINS, Barbara A., U.S. Geological Survey, Menlo Park, CA 94025, AMOS, Richard T., Earth and Environmental Sciences, University of Waterloo, Waterloo, ON N2L 3G1, Canada, BAEDECKER, Mary Jo, U.S. Geological Survey, Emeritus, Reston, VA 20192 and COZZARELLI, Isabelle M., U.S. Geological Survey, National Research Program, Eastern Branch, Reston, VA 20192, babekins@usgs.gov

Biodegradation of crude oil contamination in the subsurface has been studied for the last 30 years at a 1979 spill site located near Bemidji, Minnesota, USA. A separate phase oil body with oil saturations of 20-70% is centered on the water table 6-8 m below the surface. Degradation of the n-alkane fraction of the oil occurs by fermentation coupled to methanogenesis. The produced CH4 and CO2 migrate away from the oil in both the gas and aqueous phases. The CH4 in the gas phase is oxidized to CO2 in the vadose zone before reaching the land surface. Depleted N2 concentrations in wells downgradient from the oil body indicate outgassing of CH4 has occurred from the aqueous phase. However, dissolved N2 is not completely depleted, which provides bounds on the amount of CH4 cycled through the aqueous phase.

A two-dimensional reactive transport model of oil degradation, production of CH4 and CO2, and outgassing of CH4 and CO2 from the aqueous phase was constrained by CO2 efflux data at the land surface and dissolved concentrations of CO2, CH4, and N2. The results predict that 86% of observed surface efflux consists of outgassed carbon that could not have cycled through the aqueous phase sampled in the monitoring wells. The separation of the effect of directly outgassed CH4 from the chemistry of the well samples suggests that there are isolated water pockets in contact with the degrading oil. These water pockets may be isolated by gas bubbles present in the oil body, which lower the relative water permeability, limiting the transport of water away from the oil. Outgassing from the aqueous phase is mostly in the form of CH4, which is consistent with CH4’s much lower solubility compared to CO2. Preferential outgassing of CH4 maintains low CH4 to CO2 aqueous concentration ratios, even when the plume is strongly methanogenic.